The technology of ammonia synthesis processes is both sophisticated and highly complex.
However, at its most simplified, an ammonia synthesis process would comprise the following. Synthesis make-up gas, containing stoichiometric quantities of hydrogen and nitrogen, is compressed and introduced into a synthesis loop where it is combined with unreacted recycle gas. The gas mixture is passed to a reaction zone where hydrogen and nitrogen are reacted catalytically at superatmospheric pressure to produce ammonia. An effluent stream is withdrawn from the reaction zone and, following a series of heat exchange steps, the bulk of the ammonia is condensed and recovered therefrom. The vapour separated from this preceding recovery step (i.e. the recycle gas), which contains unreacted hydrogen, nitrogen and inert contaminants (typically methane and argon), is recompressed and returned to the reaction zone to mix with the synthesis make-up gas. A purge gas stream is vented from the synthesis loop in order to ensure that the level of inert contaminants entering the reaction zone does not exceed a pre-determined concentration. It is known that the presence of inert contaminants in the loop circulation system reduces the partial pressure of the hydrogen and nitrogen reactants, deleteriously affecting ammonia production.
The concentrations of unreacted hydrogen and nitrogen in the purge gas stream are equivalent to those in the recycle gas stream. It is normal commercial practice to recover hydrogen from the purge stream and return it to the synthesis loop, for economic reason.
Various methods of recovery of hydrogen from the purge gas stream have been proposed, exemplary of which are cryogenic or adsorption processes. One process being increasingly favoured involves a membrane permeation process.
The membrane permeation process relies on the differentials in permeation rates that exist for different gas molecules passing through a semi-permeable permeation membrane. The permeated gas is enriched with the more permeable components and conversely, the unpermeated or residue gas is enriched with the less permeable components.
In the prior art membrane permeation process, the purge gas stream is contacted in a permeator comprising a vessel having a permeation membrane contained therein. The permeator would structurally be like that described by Gardner et al. in Chemical Engineering Progress, October, 1977, pp. 76-78. The membrane utilized exhibits selectivity to the permeation of hydrogen therethrough. The commonly employed membrane material for this purpose are cellulose acetate and polysulphone, because of their high permeability to hydrogen. The membrane may be in the form of a bundle of hollow fibers. The fibre bundle is encapsulated at each end in epoxy tube sheets and the entire assembly is encased in a pressure vessel. The vessel forms a gas inlet, a permeate outlet and a stripped or residue gas outlet.
It has been found that the residual ammonia in the purge gas stream adversely affects the aforementioned commonly used prior art permeation membranes, leading to the eventual chemical decomposition thereof. Therefore, it has been necessary to subject the purge gas to an ammonia removal step prior to the hydrogen permeation step. Typically, the maximum ammonia tolerance limit of the cellulose acetate membrane is only about 10 psi ammonia partial pressure. Thus the ammonia removal step seeks to reduce the ammonia concentration to less than 0.5 mole % for a permeator operated at 2000 psia pressure.
In U.S. Pat. No. 4,172,885, E. Perry discloses a membrane permeation process for hydrogen recovery from the ammonia synthesis purge gas stream wherein the ammonia is removed from the stream by an adsorption or absorption pretreatment. The exemplary removal process is a water scrubbing treatment.
An alternative method for ammonia removal pretreatment is advanced by Doshi et al in U.S. Pat. No. 4,645,516. There a pressure swing adsorption treatment is employed in combination with a permeation membrane separation technique.